Abstract
Upconversion nanocrystals (UCNs)‐embedded microarchitectures with luminescence color transition capability and enhanced luminescence intensity under extreme conditions are suitable for developing a robust labeling system in a high‐temperature thermal industrial process. However, most UCNs based labeling systems are limited by the loss of luminescence owing to the destruction of the crystalline phase or by a predetermined luminescence color without color transition capability. Herein, an unusual crystal phase transition of UCNs to a hexagonal apatite phase in the presence of SiO2 nanoparticles is reported with the enhancements of 130‐fold green luminescence and 52‐fold luminance as compared to that of the SiO2‐free counterpart. By rationally combining this strategy with an additive color mixing method using a mask‐less flow lithography technique, single to multiple luminescence color transition, scalable labeling systems with hidden letters‐, and multi‐luminescence colored microparticles are demonstrated for a UCNs luminescence color change‐based high temperature labeling system.
Highlights
Extending the use of this optical property changing principle, we propose an optical labeling system with cryptographic multiple luminescence color, patterns, and letters for commercial high-temperature thermal processes by combining an additive color mixing method and a digital micro-mirror device (DMD)-based mask-less lithography technique
We further studied the effect of the SiO2 NPs on the spectral changes of upconversion nanocrystals (UCNs) (Composition of the precursor resin is given in Table S2, Supporting Information)
To understand the luminescence color change of the UCNs corresponding to the crystal phase transition, we considered four factors: crystal field, cross relaxation, nonradiative relaxation, and the electronic properties of the hexagonal NaREF4, cubic NaREF4, and hexagonal apatite phases of the lanthanide UCNs (Figure 4a)
Summary
NaREF4 and hexagonal apatite phases, energy transfer mechanisms of each UCNs phase are proposed (Figure 4d). Cross relaxation (i.e., close interionic distance of Er3+) and nonradiative relaxation (i.e., high phonon energy of Er3+) induced the red luminescence color. The upconversion luminescence spectrum indicates that the ratio between the green and red luminescence intensities for the hexagonal apatite phase increased as compared to that for the hexagonal NaREF4 (Figure 2b). The blue UCNs (i.e., Tm3+-doped UCNs) with/without SiO2 NPs. followed the same crystal phase transition (hexagonal to hexagonal apatite or cubic phase) as that of the yellow UCNs (i.e., Er3+doped UCNs) after annealing at 900 °C (Note S7, Supporting Information). We envision that the strategy of SiO2 NPs-promoted luminescence color change of UCNs combined with the described additive color mixing method is a effective means to label information through stepwise alteration of the luminescence color in a high temperature industrial thermal process
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